Differential pumps



Dec. 12, 1961 .1. MERCIER DIFFERENTIAL PUMPS Filed March 22, 1957 2 Sheets-Sheet 1 5? 59 Jean/$522221 BY- flwr/J ATTORNEYS Dec. 12, 1961 J. MERclER 3,012,509

DIFFERENTIAL PUMPS Filed March 22, 1957 2 Sheets-Sheet 2 12/ w w YQQ 108 1 05 [04 1/5 4 I Xx X ,0? m. mm" o o INVENTOR flan Mefcbefi ATTORN EYS Unite tates This invention relates to the art of pumps more particularly or the double acting differential reciprocating type.

It is among the objects of the invention to provide a pump of the above type which is relatively simple in construction and dependable in operation and which will provide continuous fluid flow in a single direction from the inlet through the outlet of the pump without the need for valves for reversing the lines connected to the inlet and outlet of the pump.

Another object of the invention is to provide a pump of the above type which will operate over a plurality of range of pressures and which will automatically decrease the volumetric output thereof with increase in pressure through the pump outlet and vice versa.

Still another object is to provide a pump in which the effort required for driving the pump piston is substantially constant, or at least maintained below a predetermined upper value regardless of the length of travel of the piston through a complete cycle of operation and the range of pressure on which it operates.

According to the invention these objects are accomplished by the arrangement and combination of elements hereinafter described and particularly recited in the claims.

This application is a continuation-impart of co-pending application Serial No. 284,521, filed April 26, 1952, now abandoned.

In the accompanying drawings in which are shown one or more of various possible embodiments of the several features of the invention,

FIG. 1 is a longitudinal sectional view of a pump according to one embodiment of the invention, 7

FIGS. 2 and 3 are views similar to FIG. 1 showing the elements of the pump in different positions based upon operating pressures differing from that to which the pump is subjected in FIG. 1.

FIG. 4 is a view similar to FIG. 1 of another embodiment of the invention, and

FIG. 5 is a view similar to FIG. 1 of still another embodiment.

Referring now to the drawings, as shown in FIGS. 1 to 3, the pump comprises a casing 50 illustratively a cylindrical body inside the bore 51 of which is slidingly mounted a piston 52, having aflixed thereto one end of a rod 53. The cylinder bore is closed at each end by a plug 54 provided with an outer packing 55, one of the plugs having a bore 56 with a packing ring 57 through which extends the piston rod 53-, the latter being connected through suitable linkage to control means to reciprocate the piston between the ends of the cylinder bore. The piston 52 defines inside the bore 51 a large compartment or chamber '58 located on the side opposed to the piston rod 53 and a smaller compartment or chamber 59 through which said rod 53 extends. A bore 60 having an inlet port 68 is provided for the admission of fluid into the casing 58 and is connected with the larger compartment 58 through a port 61 provided with a non-return ball valve 62 against which a spring reacts so as to permit flow of fluid in a single direction, i.e., from the bore 60 towards the compartment 58. The bore 6% is also connected with the smaller compartment 59 through a port 63 provided with a non-return valve 64 also of the spring-and ball type, permitting flow of fluid in a single direction, i.e., from the bore 60 towards the compartment 5 9.

atent The bore 60 is also connected through a port 65 with a safety device that is adapted to prevent any dangerous at its outer end by means of an adjustable screw plug 67 against which one end of a spring 68 reacts. The other end of spring 68 reacts against a ball 69 to hold the latter against a seat formed around a passageway 70 communicating with the compartment 68, the whole arrangement forming a safetyvalve which may be set to operate at a desired pressure by adjustment of screw 67.

A delivery bore 71 having an outlet port 71 is provided inside the body 50 and communicates through a port 72 with an auxiliary bore 73 formed inside the casing 58 in parallel with the main bore 51, the port 72 being located at one end of said bore 73. p

A slide valve designated as a whole by the reference number 74, is slidably mounted in said auxiliary bore 73. The valve 74 comprises a relatively short piston head '75 adjacent port 72 and a relatively long piston head 76, the two pistons being connected by a smaller diameter portion which defines an intermediate annular gap 77. A small wedge 75 is located at one end of the bore 73 against which the piston 75 may abut so that the port 72 will always remain open. A spring 78 housed inside the bore 73 is compressed between the longer piston head 76 and an adjustable threaded plug 79 screwed into the end of the bore remote from the port 72, rotation of said plug permitting adjustment of the tension of spring 78. An air vent 79 formed preferably inside the plug 79 connects with the atmosphere the portion of the bore 73 housing the spring 78 in order to prevent any undesired compression or expansion of air inside the chamberdefined by the said bore. A passageway 80 provided inside the casing 50 extends between the outer end of the larger compartment 58 and a port 81 formed in the auxiliary bore 73 at a point registering permanently with the annular gap 77.

Another passageway 82 provided inside the casing 58 connects the first mentioned passageway 80 with a port 83 opening into the auxiliary bore 73 in registry with the first mentioned port 72 while a non-return valve 84 of the ball-and-spring type in said passageway 82 permits fluid to flow only in direction leading from the passageway 80 through passageway 82 and port 83 to port 72.

A longitudinal bore 85 provided in the casing 50 connects the outer end of the smaller compartment 59 with a port 86 opening inside the auxiliary bore 73. A nonreturn valve 87 of the ball-and-spring type is located inside the bore 85 in order to prevent flow of fluid in the direction leading from the smaller compartment 59 towards the port 86.

A port 88 leads into the bore 85 at a point located between the ball of the valve 87 and the compartment 59 and communicates with an annular groove 89 in the bore 73. The groove 89 communicates through a channel 90 with the delivery bore 71 while a non-return valve 91 of the ball-and-spring type is located at the end of said channel 90 so as to permit fluid to flow only in the direction leading from the channel 90 to the delivery bore 71.

The slide valve 74 is positioned in the auxiliary bore 73 so as to move between a first position illustrated in FIG. 1 in which the piston head 76 closes the port 86 and the groove 89 with respect'to annular gap 77, to a second position illustrated in FIG. 2 in which said piston head 76 still closes the groove 89 while the port 86 registers with the annular gap 77 and to a third position (FIG. 3) in which the port 86 and the groove 89 are both uncovered and open into the annular gap 77.

The fluid-admitting bore 60 is connected through suitable conduits with a fluid container (not shown) while the delivery bore 71 is connected with a delivery conduit (also not shown).

The pump thus described, is adapted to produce a volumetric output that is greater when the pressure inside the outlet port 71' is lower and reversely, this being obtained while exerting a stress on the piston rod 53 that is substantially constant and remains in all cases lower than a predetermined maximum value.

In order to explain the operation of the pump, V will be considered the maximum volume of the larger compartment 58 and v the maximum volume of the smaller compartment 59.

The spring 78 is compressed to such a degree by adjustment of plug 79 that the slide valve 74 will occupy the first position illustrated in FIG. 1 when the pressure in the delivery pipe is less than P while said slide valve occupies the second position (FIG. 2) when said pressure is between P and F and the third position (FIG. 3) when said pressure rises above P P and P being pre determined values of the pressure and P being smaller than P2.

During operation, and when the delivery pressure is less than P, the slide valve 74 occupies the first position (FIG. 1).

In this case, when the piston 52 moves from the left hand side to the right hand side of the bore 51 (FIG. 1), the larger compartment 58 increases in volume and a volume v of fluid is delivered through the output channel 71 after passing through the bore 85, the port 88, annular groove 89, bore 90 and through the valve 91. It should be noted that the fluid can pass neither through the valve 64 nor the valve 87.

During the subsequent stroke of the piston 52 from the right hand side to the left hand side of the bore 51, which causes the volume of the larger compartment 53 to decrease, a volume V of fluid is delivered into the delivery bore 71 through the passageway 80 and 82 and the ports 83 and 72.

When the slide valve occupies the first position illustrated in FIG. 1 the pressure in the delivery bore 71 is less than P and the total volume of fluid delivered during the reciprocation of the piston is V+v.

When the pressure on the delivery side is between P and P the slide valve 74 occupies the second position illustrated in FIG. 2. In this case, when the piston 52 moves towards the right hand side so as to increase the volume of the larger compartment 58, a volume v of fluid is delivered into the delivery bore 71, through the bore 85, the port 88, the annular groove 89, the bore 90 and the valve 91 as previously described. Conversely, during the movement of the piston towards the left hand side and the consequent reduction of the volume of the larger compartment 58, the fluid delivered by said compartment passes through the passageway 80, the annular gap 77, port 86, through the valve 87 into the bore 85, the fluid passing through said bore 85 entering the smaller compartment 59. The difference in volume V-v between the smaller and the larger compartment is consequently delivered into the delivery bore 71 through the bore 82, the port 72 and/or through the bore 80, the groove 89 and the bore 90.

During a complete reciprocation, the total volume delivered is thus v+V-v=V.

When the delivery pressure is higher than P the slide valve 74 occupies the third position illustrated in FIG. 3. When the piston 52 moves towards the right and the larger compartment 58 increases in volume, the fluid delivered from the smaller compartment 59 passes through the bore 85 and enters the annular gap 77 through the groove 89 and thence through the passageway 80 back into the larger compartment 58.

During the return travel of the piston 52 from the right towards the left which causes the volume of the larger compartment 58 to decrease, the fluid driven out of said compartment passes through the bore 80 and the annular gap 77 and through the ports 86 and/or 88 enters the bore and flows into the smaller compartment 59. The difference in volume Vv between the larger and the smaller compartments is consequently sent from the passageway 80 into the delivery bore 71 through the bore 82, and the port 72 and/ or the groove 89 and the bore 90. During a complete reciprocation of the piston, the total volume delivered is thus equal to V-v.

When, with the slide valve 74 in this third position, the delivery pressure that is higher than P reaches a maximum value of P above which there exists danger of injury to the device, the pressure of the fluid inside the compartment 58 is capable of opening the safety valve 69, the tension of spring 68 being set the desired relief pressure. The fluid thus relieved flows through passageway 65 into the bore 60 and the pressure at the delivery end remains at its value P The pump just described with reference to FIGS. 1 to 3, provides for delivery of a fluid output that is less when the delivery pressure is greater and it is thus applicable to hydraulically controlled arrangements, the parts of which are to be shifted at times over a long path against a low resistance and at times over a short path against a larger resistance. The pump described meets automatically the requirements of the arrangement at any moment and is adapted to produce an increased pressure with a decreased output and reversely.

The maximum stress to be exerted on the piston rod emains however substantially the same for all three positions of the slide valve 74. In the first position (FIG. 1) this maximum stress is proportional to VP,, in the second position (FIG. 2) to VP; and in the third position (FIG. 3) to (V--v)P It is suflicient suitably to select the spring 78, the cross section of the annular gap 73, the diameter of the piston 52 and the diameter of the piston rod 53 in order that VP =vP =(V-v)P and consequently the maximum stress is substantially the same for the three positions of the slide valve which leads to similar values of the work applied to the piston in all cases; for the complete reciprocation of the piston the energy expended is identical in the three cases during the actual pumping operation.

In the embodiment of FIG. 4 the pump body is sub-divided by a piston 101 into a larger compartment 102 and a smaller compartment 103. The rod 104 of the piston 101 comprises a tubular member, the bore 105 of which opens into a chamber 106 provided inside the piston 101. A port 107 coaxial with the chamber 106 and of a smaller diameter connects said bore 106 with the larger compartment 102 inside the cylinder.

An inner piston 108 is slidably mounted inside the bore 105 of the main piston rod 104 and has a smaller diameter portion 109 that defines an annular shoulder 110 adjacent the head 108 of the piston 108. An annular gap 111 is thus provided between the portion 109 of the inner piston and the inner surface of the bore 105 associated with the chamber 106. One or more ports 112 are provided in the wall of the tubular piston rod 104 in order to connect the annular gap 111 with the smaller compartment 103 of the cylinder.

The portion 109 of the inner piston 108 is normally urged against the edge of the port 107 which defines a seat, by a spring 113 that is fitted inside the bore 105 of the rod 104 between the inner piston 108 and a plug 114 screwed into the end of the rod 104 to the desired extent in order to adjust the tension of the spring 113.

A vent 115 connects with the atmosphere the inside of the bore 105 on the side of the inner piston 108 that faces the plug 114, this vent extending through the plug and preventing any possible over-pressure or drop in pressure inside said bore.

An inlet port 116 is provided for admitting fluid into the bores 117 and 118 in pump body 100, said bores 117,

113 leading respectively into the outer ends of the larger compartment 102 and the smaller compartment 103 respectively, a non-return valve 119 being mounted in each bore 117 and 118 to permit fluid to flow in a single direction leading from each bore into the corresponding compartment.

An outlet port 126 is provided in the pump body 100 for the delivery of the fluid and leads into the two bores 121 and 122, which in turn lead into the outer ends of the larger compartment Th2 and the smaller compartment 103 respectively, while a non-return valve 123 is mounted in each bore 121, 122 in order to permit flow of fluid only in the outward direction from each compartment into the corresponding bore.

As before, V will be considered the maximum volume of the larger compartment 1'82 and v the maximum volume of the smaller compartment 103.

The area of the annular shoulder 11% is smaller than the terminal area of the portion 169 of the inner piston, said latter area being defined by a circle, the circumference of which is constituted by the line of contact between the said portion 109 and the edge of the port 107. P is the pressure inside the compartment 192 and acting on the end of the portion 109 that is required to shift the inner piston ltltl away from the port 107 against the action of the spring 113, while a pressure P that is higher than R; is required in the compartment 103 to react against the shoulder 110 to shift the piston 108 away from the port 107 against the action of the spring 113.

When the pressure at the delivery end is lower than P the piston 108 remains always in contacting relationship with the seat formed at the periphery of the port 107. During the travel of the piston 191 towards the right side, the larger compartment 102 increases in voltime and a volume v of fluid is sent through the bore 122 into the delivery or outlet port 120. During the return stroke of the piston 1&1 which causes the volume of the larger compartment 102 to decrease, a volume V of fluid is sent through the bore 121 into the delivery or outlet port 120.

During a cycle of operation the volume fed by the pump is thus equal to V-l-v.

When the delivery pressure is between R; and P and the piston ltll is moved towards the right by suitable reciprocating means (not shown) connected through link 104' to the rod 104, the larger compartment 162 increases in volume and a volume v of fluid is sent through the bore 122 into the delivery or outlet port 12%, the pressure inside the compartment 1% being lower than the pressure P required for shifting the piston Ill-3 away from its seat.

During the return stroke of the piston 101 and the consequent reduction in volume of the larger compartment 1132, the pressure inside the compartment 102 that is higher than P is now sufflcient to shift the inner piston 1G8 away from the port 167 so that a certain amount of fluid passes out of the compartment 102 into the compartment 133. The pressure inside the compartment 163 reaches the value P or above. This pressure acting on the annular shoulder 110 assists in shifting the portion 199 away from its seat. This cuts out any loss of head and provides for excellent efl'iciency. The difference V-v between the volumes of the two compartments is consequently forced through the bore 121 into the delivery or outlet port 12%. During one complete reciprocation, the volume fed by the pump is thus equal to Vv+v=V.

When the delivery pressure is higher than-P during the travel of the piston 101 towards the right, the larger compartment 1112 increases in volume and the pressure inside the compartment 193 that is higher than the pressure P is sutficient for shifting the piston 108 away from the port 167' and consequently the fluid passes through the bores 112 and 197 from the smaller compartment into the larger compartment without any substantial loss of head by reason of the fact that the pressure inside the compartment 102 reaches a value that is higher than R; and adds in the complete uncovering of the port 167.

During the return stroke which causes the volume of the larger compartment to decrease, since the pressure inside the compartment M2 is higher than P and consequently than P it is suflicient to urge the piston 108 away from the port 1&7 and consequently some fluid passes back through the bores 107 and 112 from the compartment 332 into the compartment 193 without any loss of head for the same reasons as heretofore described. The diflerence V--v is consequently sent through the bore 121 into the delivery or outlet port 126. During a complete recipr cation, the volume fed by the pump is thus equal to V-v.

Thus, the pump is capable of operating for three ranges of pressure and produces automatically, according to the pressure at the delivery or output end, an increase in pressure at the expense of the output and reversely, while the stress exerted on the piston rod remains substantially the same or is at least less than a predetermined limit value.

A safety valve (not shown) and that is tensioned for instance so as to operate at a predetermined pressure value that is higher than P may be provided on a terminal wall of the larger compartment 102.

In the embodiment shown in FIG. 5, the body of the pump is sub-divided by a piston 141 into a larger compartment 142 and a smaller compartment 143 through which the piston rod 144 extends.

A first bore 145 provided inside the piston 141 extends between the compartments 142 and 14-3 and is provided with a valve 146 of the ball-and-spring type which prevents the flow of fluid in the direction leading from the smaller compartment 143 into the larger compartment 142. The reverse flow is allowed provided the pressure inside the compartment 142 is higher than the value P which is necessary for shifting the ball away from its seat against the action of the spring 146.

A second bore 147 also extends inside the piston 141 between the two compartments T42 and 143 and is provided with a valve 148 of the ball-and-spring type that prevents the flow of fluid in the direction leading from the larger compartment 142 towards the smaller compartment 143 while the flow in a reverse direction is allowed, provided the pressure inside the compartment 143 is higher than the value P required for shifting the ball of the valve 148 away from its seat against the action of its spring 148'. This value P is selected so as to be higher than P To accomplish the desired operation of the valves 46, 148, a spring may be selected for the valve 148 that is stronger than the spring of the valve 146 or the surface of the ball of the valve 148 that is submitted to pressure may have an area smaller than the corresponding area of the pressure-submitted surface of the ball of the valve 146 or these two arrangements may be combined.

An inlet port 150 is provided for admitting fluid into the body 144) and leads into two bores 151, 152 connected respectively to the outer ends of the larger compartment 142 and the smaller compartment 143, a non-. return valve 153 being located in each bore 151, 152 in order to allow the fluid to flow only in the direction leading from each bore towards the corresponding compartment.

An outlet port 154 is provided for the delivery of fluid into body 140 and leads into two bores 155 and 156, connected respectively to the outer ends of the larger compartment 142 and the smaller compartment 143, a non-return valve 157 being located in each bore 155, 156 in order to allow the fluid to flow only in the direction leading from each compartment into the corresponding bore.

1 The operation is the same as that described with respect to HG. 6. V remains always the maximum'volume of the larger compartment and v the maximum volume of the smaller compartment. When the delivery pressure is less than P and consequently less than P the fluid pre sure acting against the valves M6 and 148 is insufficient to move the latter off their seats during reciprocation of the piston 141. Hence, the volume fed during one complete reciprocation is equal to V+v.

When the delivery pressure is between P and P and the piston moves to the left, as the pressure of the delivery fluid is greater than P ball valve 146 will move of? its seat again the action of spring 1 :6 for flow of fluid from chamber 14-2 into chamber 143. However, with the delivery pressure below P when the piston 141 moves to the right, the pressure in chamber 143 will be insufiicient to move valve 143 off its seat against the action of spring 148. Hence, the volume fed by the pump is equal to V -v+v or V.

When the delivery pressure is greater than P and the piston moves to the left, ball valve 146 will move off its scat against the action of spring 146' for flow of fluid from chamber 142 into chamber 143. When the piston moves to the right, ball valve 148 will move off its seat against the action of spring 148' for flow of fluid from chamber 143 into chamber 142. Hence, the volume fed by the pump is equal to Vv+v-v=V-v.

As in the preceding examples, the pump may operate automatically and in accordance with the delivery pressure, while the controlling stress exerted on the pump remains substantially constant and at any rate less than a predetermined upper limit.

It is also possible to provide in the case illustrated in FIG. 5, a safety valve (not shown) in order to retain the pressure inside the larger compartment 142 below a pre' determined safety value.

As many changes could be made in the above constructions, and many apparently widely different embodiments of this invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having thus described by invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. A pump comprising a casing having a bore with a piston slidably mounted therein and adapted to be reciprocated in such bore, said piston defining a chamber in said bore on each side thereof, a piston rod affixed at one end to said piston and extending through one of said chambers, whereby the operative cross section of said last named chamber is smaller than that of the other chamher, an inlet port, conduit means connected between said inlet port and said chambers adjacent the ends thereof remote from said piston for admitting fluid into said chambers, an outlet port, conduit means connected between said outlet port and said chamber adjacent the ends thereof remote from said piston for discharge of fluid from said chambers, one-way valve means associated with said conduits and said inlet and outlet ports to permit fluid to flow in direction into and out of said chambers respectively, conduit means connecting said two chambers, 21 valve means unit repsonsive to two different amounts of the pressure of the fluid flowing through the outlet port, controlling said last named conduit means and normally retaining the latter closed when such pressure is less than the first predetermined amount and permitting flow through said last named conduit means from said larger chamber into said smaller chamber when said pressure is greater than such first predetermined amount, but less than a second predetermined amount and permitting flow from either chamber into the other when such pressure is greater than said second predetermined amount.

2. A pump comprising a casing having a bore with a piston slidably mounted therein and adapted to be reciprocated in such bore, said piston defining a chamber in said bore on each side thereof, a piston rod aflixed at one end to said piston and extending through one of said chambers, whereby the operative cross section of said last named chamber is smaller than that of the other chamber, means for admitting fluid into the outer extremities of both chambers, delivery means for the fluid opening into the outer extremities of both chambers, non return valves in the said admission means and in the said delivery means, conduit means connecting said two chambers, a valve unit responsive to two dififerent amounts of the pressure of the fluid in said chambers, controlling said last named conduit means and normally retaining the latter closed when such pressure is less than the first predetermined amount and permitting flow through said last named conduit means from said larger chamber into said smaller chamher when said pressure is greater than such first predetermined amount, but less than a second predetermined amount and permitting flow from either chamber into the other when such pressure is greater than said second predetermined amount.

3. A double-acting reciprocating piston pump comprising a housing having a closed cylinder bore therein, said housing having an aperture through one end wall of said bore; a piston and piston stem assembly reciprocable in said bore and defining therein a first and second working space, said piston stem extending outward through said aperture; means affording a valve controlled inlet connected to both working spaces; rncans affording a valve-controlled discharge connection from each working space, said piston having an opening extending axially through said assembly; check valve means controlling flow through said opening and arranged to inhibit flow from the smaller to the larger working space; a motor carried by the assembly and biased to close said valve and urged by discharge pressure toward a position in which said valve may open to permit flow from the larger to the smaller working space.

4. A pump comprising a casing having a bore with a piston slidably mounted therein and adapted to be reciprocated in such bore, said piston defining a chamber in said bore on each side thereof, a piston rod aflixed at one end to said piston and extending through one of said chambers, whereby the operative cross section of said last named chamber is smaller than that of the other chamber, an inlet port, conduit means connected between said inlet port and said chambers adjacent the ends thereof remote from said piston for admitting fluid into said chambers, an outlet port, conduit means connected between said outlet port and said chamber adjacent the ends thereof remote from said piston for discharge of fluid from said chambers, one-way valve means associated with said conduits and said inlet and outlet ports to permit fluid to flow in direction into and out of said chambers respectively, conduit means connecting said two chambers, valve means controlled by the pressure of the fluid flowing through the outlet port, controlling said last named conduit means and normally retaining the latter closed when such pressure is less than a first predetermined amount and permitting flow through said last named conduit means from said larger chamber into said smaller chamber when said pressure is greater than such first predetermined amount, but less than a second predetermined amount and permitting flow from either chamber into the other when such pressure is greater than said second predetermined amount, said last named conduit means being in said piston, the valve means controlling said last named conduit having resilient means reacting against said valve means normally to retain said conduit closed when the pressure on the fluid in said outlet port is less than said first predetermined value, said valve means including two surfaces against which the fluid under pressure will react, said surfaces being exposed respectively to the fluid under pressure in said two chambers when the conduit is closed, the fluid pressure against the surface exposed to the pressure of the fluid in the larger chamber opening said conduit against the action of said resilient means when the pressure reacting against such surface exceeds said firstpredetermined amount and said fluid pressure against the other surface opening said conduit only when said pressure has exceeded said second predetermined amount.

5. A pump comprising a casing having a bore with a piston slidably mounted therein and adapted to be reciprocated in such bore, said piston defining a chamber in said bore on each side thereof, a piston rod atfixed at one end to said piston and extending through one of said chambers, whereby the operative cross section of said last named chamber is smaller than that of the other chamber, an inlet port, conduit means connected between said inlet port and said chambers adjacent the ends thereof remote from said piston for admitting fluid into said chambers, an outlet port, conduit means connected between said outlet port and said chamber adjacent the ends thereof remote from said piston for discharge of fluid from said chambers, one-way valve means associated with said conduits and said inlet and outlet ports to permit fluid to flow in direction into and out of said chambers respectively, conduit means connecting said two chambers, valve means controlled by the pressure of the fluid flowing through the outlet port, controlling said last named conduit means and normally retaining the latter closed when such pressure is less than a first predetermined amount and permitting flow through said last named conduit means from said larger chamber into said smalle chamber when said pressure is greater than such first predetermined amount, but less than a second predetermined amount and permitting flow from either chamber into the other when such pressure is greater than said second predetermined amount, said piston having a bore therethrough defining said last named conduit means, said bore being of reduced diameter at the end thereof adjacent the larger chamber and defining a valve seat, said piston rod having a bore axially aligned with said piston bore and in communication therewith, the valve means controlling said last named conduit means comprising a plunger defining a valve seat closure at one end and having a head of enlarged diameter at its other end defining a shoulder of smaller cross sectional area than that of the reduced diameter portion of said piston bore, said plunger being slidably mounted in said bores, resilient means in said piston rod bore reacting against said plunger normally to retain the closure end thereof against said seat, said piston rod having a passageway therethrough from said smaller chamber into the portion of the piston rod bore between the closure end and head of said plunger.

6. The combination set forth in claim 5 in which said resilient means is a spring conformed to retain the closure end of said plunge against said seat when the pressure of the fluid through the outlet port is less than said first predetermined amount and the cross sectional area of the reduced portion of the piston bore is such that the fluid under pressure in said larger chamber will overcome the force exerted-by the spring against the plunger to move the latter off its seat when the pressure is greater than said first predetermined amount, while the cross sectional area of said shoulder subjected to the pressure of the fluid in said smaller chamber when said closure end is seated is such as to preclude movement of said plunger when the pressure is less than said second predetermined amount, both of said areas being such as to overcome the force exerted by said spring to move the closure end of the plunger ofi its seat when such pressure exceeds said second predetermined amount.

7. The combination set forth in claim 6 in which means are provided to adjust the tension on said resilient means.

References Cited in the file of this patent UNITED STATES PATENTS 154,468 Flanders Aug. 25, 1874 1,170,873 Brown Feb. 8, 1916 1,196,920 Astrom Sept. 5, 1916 2,246,932 Collins June 24, 1941 2,362,139 Kelly Nov. 7, 1944 2,510,150 Stephens June 6, 1950 2,749,845 Bahniuk June 12, 1956 FOREIGN PATENTS 10,111 Norway Nov. 18, 1901 58,441 Netherlands Oct. 15, 1946 76,925 Germany Aug. 25, 1894 

